After a well has been drilled such as for discovery and production of petroleum products, wireline controlled apparatus is frequently employed to conduct various downhole installation, retrieval and servicing operations. Wireline equipment is utilized to install and retrieve a wide variety of downhole tools such as packers, gas lift valves, downhole safety valves, bottom hole pressure sensors and the like. Wireline equipment is also frequently utilized to run various well servicing tools such as for cleaning and treating production tubing.
At times it is desirable to establish a seal about the wireline to permit well servicing operations to be conducted within the wellhead and free of well pressure above the blowout preventer (BOP). More importantly, wireline BOP's are required to accommodate various emergency situations and alleviate any dangerous condition that might otherwise occur. During wireline operations the wireline BOP is typically static in an open position which allows the wireline to freely traverse the wellhead of the well without interference with any portion of the wellhead structure including the safety equipment of the wellhead.
At times during well servicing operations it will be necessary to close and seal the wireline BOP about the wireline. For example, in the event a kick (pressure surge) develops in the open hole or in the casing of the well, the wireline BOP is closed in response to the kick to develop a seal about the wireline to contain well pressure and prevent a blowout. With well pressure thus contained kill fluid can be pumped into the well below the seal established by the wireline BOP to shut in the well to thus permit other well servicing operations to be safely conducted. The wireline BOP may also be closed to temporarily contain the well pressure while bleeding well pressure off the lubricator of the wireline equipment to thus permit replacement of the packoff elements of the lubricator. In some cases it becomes necessary to remove, add or make repairs to the riser or cable of the wireline well servicing equipment. In this case the wireline BOP is typically closed and sealed about the wireline cable to thus contain the well pressure and permit cable repairs to be made above the level of the wireline seal. Most critically, wireline BOP's are shut automatically or manually in the case of failure of the well control system above the level of the BOP and thus must develop an efficient seal about the wireline to prevent any leakage from occurring.
Typically, wireline sealing elements are formed by two elastomer faced metal rams which have been contoured to fit a particular size of wireline cable. These rams are positioned in opposed relation and are actuated such that they accomplish centering of the wireline and establish sealing engagement with the wireline at a location that is centrally of the BOP housing. The opposed rams are closed manually or hydraulically (with mechanical backup) to develop a positive seal about the wireline to contain the well pressure and thus shut in the well. When the BOP is activated, the elastomer faces of the rams will seal around the outer surface of the cable by extruding the elastomeric material into the interstitial spaces between the armor strands of the wireline. When pressurized grease is present between the rams of a BOP its pressure, acting on the pressure responsive force area of each ram tends to oppose the closing force of the ram. This problem can be overcome to some extent by venting excess grease pressure from the faces of the rams as is taught in Applicant's earlier U.S. Pat. No. 4,938,290. It is desirable to provide for utilization of the grease pressure for enhancing the sealing capability of the rams.
Ram type BOP's are designed to seal with the wireline cable in a static position. Especially for the reason that the extruded elastomer material in the interstices of the cable strands is easily torn away by relative cable movement, it is necessary therefore to always stop movement of the wireline before the rams are closed about the wireline to effect sealing. Short lengths of wireline cable can be stripped through the BOP as needed to repair a stranded cable but the amount of elastomeric material that can be worn away by such stripping without resulting in leakage of the BOP is limited. Pulling an appreciable length of wireline cable through the BOP will induce severe wear to the elastomeric seals of the rams and can cause damage to the rams as well, thus causing a more serious failure.
The highly pressurized elastomeric seal is prevented from extrusion by the close metal-to-metal fit of the ram faces and due to the fact that differential pressure moves the cylindrical rams tightly against the upper wall of the BOP housing. When the pressure of the elastomer against the surrounding surfaces of the BOP body exceed differential pressure a leak-tight seal will be effected. Wireline BOP's are designed to seal against well pressure in only one direction and therefore care must be taken to insure that they are not installed upside-down when a single set of rams is employed because an invested BOP will not hold well pressure. Single ram BOP's are only installed upside-down to contain pressure injected from above. Thus inverted single ram BOP's are virtually always used above a BOP that can contain well pressure.
It is difficult to move the rams of wireline BOP's when the rams are closed against high differential pressures. Wireline BOP's must therefore be provided with a bypass that is used to equalize the pressure across the rams before opening of the rams is initiated. After equalization of well pressure across the rams has occurred there will be a partial relaxation of the sealing contact of the elastomeric seals with the wireline, thus reducing the extrusion of the elastomeric sealing material about the wireline and as a consequence, reducing the friction between the sealing material and the wireline. This effective reduction of friction makes the rams much easier to open. To allow a controlled equalization of pressure, the BOP is typically fitted with a pressure equalizing system that is externally controllable to enable efficient operation by service personnel.
Manual BOP's are typically operated by selectively turning two ram operator handles on opposed sides of the apparatus to open and close the opposed rams. Manual BOP's are available in a number of sizes and ratings. Regardless of the pressure rating, they are normally used at lower pressures for standard service. Manual BOP's are typically of lighter weight and are less expensive as compared to BOP's having hydraulically energized rams. Because operating personnel must gain physical access to manual BOP's for opening and closing the rams, these persons are typically in a more dangerous location, i.e. immediately adjacent the wellhead, during BOP operation. Also, less ram pressure can be applied with manual BOP's than with hydraulically energized ram BOP's. In larger sizes, as commonly used for open hole work, the manual BOP offers adequate protection and is considerably lighter than a hydraulically energized BOP.
Hydraulic ram BOP's are opened and closed by hydraulic pressure acting on ram operating pistons that are located within hydraulic cylinders. Hydraulic BOP's are also typically provided with operating handles and stems which are used for manual backup. A hydraulic BOP can generally be closed manually but usually must be opened hydraulically. The manual operating stems must be backed out manually to allow hydraulic piston movement before the rams can be opened hydraulically.
The rams and sealing elements of wireline BOP's have grooves that are sized for the wireline cable diameter being employed. As the rams are closed, the wireline cable is guided by the rams or other cable guide elements into the grooves of the sealing elements. Wireline BOP's are provided with "integral guide" rams that prevent cable damage as the cable is guided and centralized during ram closure. If the rams of hydraulic wireline BOP's are to be left closed for a long period of time or in case of hydraulic failure, the manual screw jacks defining the manual operating stems can be used to secure the rams in the closed position and prevent inadvertent piston and ram movement. To open a hydraulic wireline BOP, the mechanical backup must be in the open position before shifting the hydraulic "selector" to its open position and hydraulically forcing the rams to their open positions by means of hydraulic fluid pressure.
Multiple ram BOP's, typically dual wireline BOP's, are utilized to provide a backup in case of failure of the primary set of sealing rams. More importantly is the fact that gas tends to migrate through the interstices between the inner and outer armor of the wireline cables. In the event that the lubricator of the wireline equipment should need to be removed for some reason, any natural gas leakage through the interstices of the standard cable could quickly present a significant problem from the standpoint of danger to personnel because of its explosive nature. To alleviate this problem, a second (tandem) BOP is typically added. This BOP is inverted (because wireline BOP's hold pressure in only one direction) and an injection port is provided between the two BOP's. High pressure grease (above well pressure) is then injected into the flow passage between the upper and lower sets of rams. Under high pressure, the grease is caused to migrate into the interstitial space between the inner and outer armor of the wireline and thereby effects a grease seal to prevent gases from escaping. In some cases a triple BOP is also installed to provide a backup in the case of primary ram failure. A triple BOP also provides a method for injecting grease between the BOP's if needed. Obviously the BOP riser becomes quite high when triple BOP's are installed in tandem. Multiple ram BOP's are available in a single forged body for lighter weight and more compact size if BOP riser height becomes a problem.
As explained above, it is frequently necessary to employ dual and sometimes triple wireline BOP's and to provide for grease injection between them in order to effect a proper wireline seal and prevent migration of gases through the interstices of the wireline. Since the flow passage between the stacked or multiple wireline BOP's is typically of significant length and is at least as great as the diameter of the flow passage through the wireline extends, a considerable volume of injected grease is necessary to fill the flow passage to accomplish efficient sealing. Also, injection of a sufficient volume of grease to fill the flow passage and develop a hydraulic seal with the wireline can require a considerable period of time. It is desirable therefore to minimize the time required to develop an adequate hydraulic seal with the wireline to thus promote the safety of the sealing operation. Obviously, employment of dual and triple ram BOP's, whether of the manual or hydraulic type, can result in considerable expense. Moreover, the stacking of dual and triple wireline BOP's ordinarily results in a BOP riser of significant height. In many cases there is little room to accommodate multiple wireline BOP's of this nature. Accordingly, it is desirable to provide a single ram wireline BOP having the capability of providing both elastomeric sealing by means of opposed rams and to provide hydraulic sealing by means of grease injection. Additionally it is desirable to provide for pressure enhanced sealing of a wireline BOP by means of the pressure of injected sealing grease.
As indicated above, to provide a wireline BOP system with the capability of accomplishing ram energized elastomer sealing with a wireline and to accomplish grease sealing of the wireline typically requires two or more wireline BOP's that are coupled to define a BOP riser with a large volume grease chamber being defined by the flow passage between them. It is desirable to provide a single ram BOP having both the capability of elastomer sealing and grease sealing with the wireline and also having the capability of being hydraulically opened and closed by a selectively operably hydraulic system and which incorporates a secondary hydraulic system for accomplishing grease pressure energized, hydraulically induced enhancement of the closing force of the opposed rams of the BOP.
Hydraulically energized wireline BOP systems ordinarily require a BOP design having a very large ram element and a special outer seal which can significantly increase the complexity and cost of a wireline BOP in comparison with more conventional BOP systems. It is desirable therefore to develop a ram element for a BOP mechanism which serves the grease injection requirement as well as BOP mechanisms having large ram elements but which is of substantially the same size as that of an existing generic wireline BOP. This feature will provide the very desirable results of (a) allowing the use of a standard sized outer seal for the rams (b) fitting of the structural components of the rams into a standard diameter ram body and (c) allowing the use of a standard diameter blowout preventer body. All of these factors materially influence the cost and thus the competitive nature of the BOP mechanism.
It is therefore a primary feature of the present invention to provide a novel wireline BOP having a controllable hydraulic system for selective opening and closing the rams such as for testing or closing under well pressure and having a grease pressure energized system for enhancing the closing force of the rams against the wireline to thus provide for grease pressure enhanced sealing thereof.
It is another feature of the present invention to provide a novel single ram wireline BOP construction having rams that are designed both for elastomer sealing with the wireline and grease sealing with the wireline and yet which utilizes rams of sufficiently small dimension as to be dimensionally comparable with a generic wireline blowout preventer.
It is also a feature of the present invention to provide a novel single ram wireline BOP, whether energized by selective hydraulic operation or by automatic well pressure responsive, piston energized operation, which achieves an efficient mechanical seal about braided wireline to efficiently contain well pressure below the rams.
It is also a feature of this invention to provide a novel single ram wireline BOP that, when closed, defines a small volume grease chamber located between spaced elastomer seals of the ram to thus provide the capability of grease injection about the wireline to thus provide an additional hydraulic seal about the wireline cable to prevent the flow of gases through the interstices formed by the armor of the cable.
It is an even further feature of this invention to provide a novel single ram wireline BOP incorporating both mechanical and hydraulic sealing and wherein hydraulic sealing is effected quickly by means of a minimal volume of injected grease.